Centuries

A century (from the Latin centum, meaning one hundred; abbreviated c.) is 100 years. Centuries are numbered ordinally in the English language along with many others (e.g. "the 7th century AD/CE"). A centenary is a hundredth anniversary or a celebration of this, typically the remembrance of an event which took place a hundred years earlier. Its adjectival form is centennial.

According to the Gregorian calendar, the 1st century AD/CE started on January 1, 1, and ended on December 31, 100. The 2nd century started at year 101, the 3rd at 201, etc. The n-th century started/will start on the year (100 × n) − 99 and ends in 100 × n. A century will only include one year, the centennial year, that starts with the century's number (e.g. 1900 is the final year of the 19th century).

Because the Gregorian Calendar does not have a year 0, purists have argued that a new century does not begin until **01.

Dionysius Exiguus of Scythia Minor introduced the anno Domini system in AD 525, counting the years since the birth of Christ.^[10] This calendar era is based on the traditionally reckoned year of the conception of Jesus of Nazareth, with AD counting years from the start of this epoch, and BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.

There is a year 0 however in the astronomical year numbering and in the ISO 8601:2004. In this case, both 1900s and 20th century have the same time span (1900-1999). However, most people assume and prefer that a new century begins with two 0s as its last digits (1700, 1800, 1900, 2000, etc.)

...
Wikipedia